During well drilling operations, especially when drilling wells intended for recovering underground oil and/or gas fields, use is made of drilling fluids intended to lubricate, clean and cool the drilling tools and the drilling head, and/or to discharge the material broken off during drilling (cleared rocks or cuttings). Drilling fluids are also used for cleaning the well. They also provide the pressure necessary for supporting the well wall before consolidation. The fluids are usually referred to as “drilling muds”. After drilling, the well walls are generally consolidated with a cement material.
During the drilling of wells, in particular during the drilling of wells intended for the production of oil and/or gas, drilling is often carried out through argillaceous rocks, in particular through shales.
The problems posed by argillaceous formations are well known. When these formations are penetrated by drilling using water-based drilling fluids, complex chemical reactions occur within the argillaceous structure by ion exchange and hydration.
These reactions result in a swelling of the clays, a disintegration or a dispersion of the argillaceous particles of the formation passed through by the drilling.
This swelling of the clays poses problems not only in the drilling walls but also in the drilling fluid and in the reservoir rock.
The expression “reservoir rock” is understood to mean the rock formation that contains the oil and/or gas to be extracted.
Due to the hydration of the clays, dispersed particles contaminate the drilling fluid and the reservoir rock, and the disintegration is detrimental to the stability of the well walls. The swelling of these clays also causes operational problems by interfering with the flow of the fluid or the passage of the drilling tool.
Along the well walls, the swelling creates protuberances, which interferes with the movement of the drilling fluid and of the drilling tools. Furthermore, the swelling may result in disintegration, creating bumps along the walls. These bumps and protuberances may create points of mechanical weakness in the well.
In the drilling fluid, the disintegrated argillaceous material is released into the fluid and presents problems of viscosity control of the fluid: the argillaceous materials, especially in the presence of a high concentration of salts (brine), have a tendency to greatly increase the viscosity. This increase in viscosity becomes detrimental and, if it is too high, the drilling tools are damaged. The well may even be rendered unusable.
Furthermore, the cleared argillaceous rocks may have a tendency to aggregate together in the drilling fluid (“bit-balling” phenomenon). Generally, it is referred to as an accretion phenomenon. The accretion may interfere with the movement of the fluids and of the tools. They may furthermore adhere to and aggregate together around the drilling head and thus block it.
The problem presented by the swelling of the clays during drilling in argillaceous formations is closely linked to the phenomena of clay/drilling fluid interactions, especially during clay-water contact.